The present invention relates to a focal point dislocation detection method of detecting a focal point dislocation generated in a converging optical system, and to an optical pickup apparatus in which the focal point dislocation detection method is employed.
Recently, there is a demand for an optical disc having a high recording density, in response to an increase in an amount of information. The high recording density of the optical disc has been achieved by increasing linear recording density in an information recording layer of the optical disc or by narrowing a track pitch of the optical disc. In response to the trend for the high recording density of the optical disc, it is necessary to have a small beam diameter of a light beam that is converged on the information recording layer of the optical disc.
In order to make the beam diameter of the light beam small, (a) the light beam, which is directed from an objective lens (a converging optical system of a light pickup apparatus for recording/reproducing the optical disc), has a high numerical aperture (NA), or (b) the light beam has a short wave length.
As to the short wave length of the light beam, it is believed that the short wave length of the light beam can be realized by using, as a light source, a bluish purple semiconductor laser, which has been developed paving the way for its commercial use, instead of a red semiconductor laser.
On the other hand, in order to realize the objective lens having the high numerical aperture, suggested is a method in which an objective lens is coupled with a semispherical lens so as to constitute an objective lens with the two lenses (a couple of lenses), thereby increasing the high numerical aperture.
In general, the information recording layer of the optical disc is covered with a cover glass so that the information recording layer can be protected from being attached by a dust or being damaged. Therefore, a light beam that has passed through the objective lens of the optical pickup apparatus passes through the cover glass, then is converged to make a focal point on the information recording layer that is underneath the cover glass.
When the light beam passes the cover glass, spherical aberration (SA) is generated. The spherical aberration is obtained by an equation (1) as follows:
SAxe2x88x9ddxc2x7NA4xe2x80x83xe2x80x83(1).
As indicated by the equation (1), the spherical aberration is proportional to a thickness of the cover glass d, and the fourth power of NA. Because the objective lens is usually so designed to compensate the spherical aberration, the light beams that has passed through the objective lens and the cover glass, has a sufficiently small spherical aberration.
However, if the cover glass has a thickness that is different from a predetermined thickness, the spherical aberration is generated in the light beam converged on the information recording layer, thereby enlarging its beam diameter, causing such a problem that correct reading and writing of the information are impossible.
Moreover, the equation (1) indicates that a larger error xcex94d of the thickness of the cover glass gives a larger error xcex94SA of the spherical aberration, thereby making it impossible to read and write the information correctly.
Furthermore, a multilayer optical disc, in which the information recording layers are laminated, has been developed for a commercial use, so as to record the information in a still higher density in terms of a thickness direction of the optical disc. For example, a DVD (Digital Versatile Disc) having two information recording layers has been developed as the multilayer optical disc. For an optical pickup apparatus for recording/reproducing the multilayer optical disc, it is necessary that the light beam is converged sufficiently small for each information recording layer of the optical disc.
For the multilayer optical disc having the plural information recording layers, thicknesses from a surface of the optical disc (surface of the cover glass) to the respective information recording layers are different from each other. Because of this, each information recording layer has respectively a different spherical aberration, which has been generated when the light beam passes the cover glass of the optical disc. In this case, for example, a difference (error xcex94SA) between the spherical aberration generated in the information recording layers adjoining each other is proportional to a layer-to-layer space t between the adjacent information recording layers, according to the equation (1).
For the DVD having the two information recording layers, used is an optical pickup apparatus that has an objective lens NA of which is as small as 0.6. Because of this, in the DVD having the two information recording layers, the error xcex94d of the thickness of the cover glass does not largely affect the error of the spherical aberration xcex94SA, even though the error xcex94d of the thickness of the cover glass becomes large to some extent, according to the equation (1).
Therefore, with a DVD apparatus provided with the conventional optical pickup apparatus having the numerical aperture NA of about 0.6, it is possible to converge the light beam sufficiently small on each information recording layer, because the error xcex94SA of the spherical aberration, which is generated due to the error xcex94d of the thickness of the cover glass of the DVD, is small.
However, even if the error xcex94d of the thickness of the cover glass is constant, the greater the NA is, the larger spherical aberration SA is generated. For example, when NA=0.85, an approximately four-time larger spherical aberration SA is generated, compared with the case where NA=0.6. Therefore, the equation (1) shows that the respective spherical aberration due to the error in the thickness of the cover glass becomes larger as the NA becomes higher, for example, when NA=8.5.
Similarly, in case of the multilayer optical disc, the spherical aberration have a greater differences (error xcex94SA) as the objective lens of the optical pickup apparatus has a larger NA, even if the layer-to-layer distance t between the adjacent information recording layers is constant. For example, when NA=0.85, a approximately four-time larger difference is generated between the spherical aberration SA, compared with the case where NA =0.6. Therefore, according to the equation (1), it is indicated that the difference between the respective spherical aberration between the respective information layers gets greater as the NA becomes higher, for example, when NA=8.5.
Thus, it is a problem for an objective lens having a high NA, which is inevitably affected by the error of the spherical aberration, that the information is read in a low accuracy. Thus, it is necessary to compensate for the spherical aberration in order to realize the high recording density by using the objective lens of the high NA.
As a method for detecting and compensating the spherical aberration, for example, the U.S. patent application, Ser. No. 09/456,414 (filed on Dec. 8, 1999) discloses an optical pickup apparatus for detecting and compensating the spherical aberration. The optical pickup apparatus takes advantages of a feature that the light beam in the vicinity of an optical axis is converged in a different position from a position where the light beam outside the vicinity of the optical axis is converged, in accordance with the spherical aberration, when the light beam is converged on the information recording layer of the optical disc.
The optical pickup apparatus disclosed in the publication, in which an optical element such as a hologram is used to separate a light beam, which is to be detected, into the light beam in the vicinity of the optical axis and the light beam outside the vicinity of the optical axis so as to detect the dislocation of the convergence of one of the light beams, when the spherical aberration is generated, the convergence of which is dislocated from the information recording layer. With the optical pickup apparatus, the spherical aberration can be compensated in accordance with a result of the detection, so as to sufficiently reduce the diameter of the light beam converged on each information recording layer of the optical disc.
Moreover, the optical pickup apparatus performs focal point positional adjustment of the optical system, for example, by halving the light beam in halves by means of the hologram and the like, so as to converge the half parts of the light beam on a two-splitting photodetector. Then, the optical pickup apparatus detects, as a signal regarding the focal point dislocation (focal error signal), a difference generated by the two-splitting photodetector so as to compensate the focal point dislocation in accordance with the signal. In general, this method is called as a beam-size method.
However, in case the optical pickup apparatus disclosed in the publication is adopted in an optical recording/reproducing apparatus in which the lens having the high NA or the light source for the light beam having the short wave length are used in order to achieve the high recording density, the spherical aberration causes an offset in the focal error signal that has been detected as above. Therefore, the diameter of the light beam cannot be sufficiently small on the information recording layer of the optical recording medium (disc), thereby causing such a problem that the information cannot be recorded/reproduced to/from the optical recording medium.
The present invention has an object to provide a focal point dislocation (focus error) detection method for detecting focal point dislocation in a converging optical system without an offset, so as to perform accurate focusing of the converging optical system onto an information recording layer of an optical recording medium, and an optical pickup apparatus using the focal point dislocation detection method.
In order to attain the above object, a focal point dislocation detection method of the present invention includes the step of detecting focal point dislocation of a converging optical system in accordance with, among light beams that has passed through the converging optical system, a light beam that corresponds to an extreme value of a curve and a region in a vicinity of the extreme value, where the curve represents a wavefront of such a state that the converging optical system is so adjusted to have an image point at which the light beam has a smallest beam diameter.
Here, when illustrated by a curve line is the wavefront of the state where the converging optical system is adjusted to have an image point (an optimum image point) having the smallest beam diameter of the light beam, the curve line have a tangent line that is approximately parallel to a tangent line of a curve line at an extreme value, where the curve line at the extreme value represents an ideal wavefront free from the spherical aberration. This indicates that a convergent point (focal point) and the optimum image point are located in an almost same position. Here, the convergent point is a point on which the light beam, which passes through the extreme value of the curve line that represents the wavefront of the case where the light beam has the optimum image point on the information recording layer of the optical recording medium.
Therefore, where the wavefront of a state where the light beam, which passes through the converging optical system, has the optimum image point, the focal point dislocation of the converging optical system can be accurately detected, without being largely affected by the spherical aberration, by performing the detection in accordance with the light beam that corresponds to the extreme value of the curve and a vicinity of the extreme value, even if the spherical aberration is generated in the converging optical system.
Because this makes it possible to optically detect the focal point dislocation of the converging optical system without the offset, it is possible to appropriately compensate for the focal point dislocation of the converging optical system. As a result, it is possible to accurately focus to have the focal point of the converging optical system of the information recording layer of the optical recording medium.
Moreover, another focal point dislocation detection method of the present invention includes the step of detecting focal point dislocation of a converging optical system in accordance with a light beam of a 60% to 85% region of a light beam effective diameter, where the light beam effective diameter, which is centered with respect to an optical axis of the light beam passing through the converging optical system including an objective lens, is regulated by a an aperture diameter of the objective lens.
Here, an optimum image point of the light beam, which passes through the converging optical system, approximately matches with a converging point (focal point) at which the light beam of the 60% to 85% region of the light beam effective diameter, which is centered with respect to an optical axis of the light beam passing through the converging optical system including an objective lens, is regulated by an aperture diameter of the objective lens.
Therefore, even if the spherical aberration is generated, it is possible to accurately detect the focal point dislocation without being significantly affect by the spherical aberration, with the above arrangement where the focal point dislocation of a converging optical system is detected in accordance with a light beam of a 60% to 85% region of a light beam effective diameter, where the light beam effective diameter, which is centered with respect to an optical axis of the light beam passing through the converging optical system including an objective lens, is regulated by a an aperture diameter of the objective lens.
Therefore, even if the spherical aberration is generated, it is possible to accurately detect the focal point dislocation without being significantly affect by the spherical aberration, with the above arrangement where the focal point dislocation of a converging optical system is detected in accordance with a light beam of a 60% to 85% region of a light beam effective diameter, where the light beam effective diameter, which is centered with respect to an optical axis of the light beam passing through the converging optical system including an objective lens, is regulated by a numerical aperture of the objective lens.
Because the focal point dislocation of the converging optical system can be optically detected without an offset, the focal point dislocation of the converging optical system may be appropriately compensated. As a result, the focal point of the converging optical system can form a focal point accurately on the information recording layer of the optical recording medium.
For a fuller understanding of the nature and advantages of the invention, reference should be made to the ensuing detailed description taken in conjunction with the accompanying drawings.